A thin film for bolometer that is used as an uncooled infrared sensor: is made of a metal thermally-isolated from a circuit board such as a Si substrate, a semiconductor or the like; and functions as an element, in which the temperature changes by absorbing the incoming infrared light, the electric resistance changes in response to such temperature variations, and the intensity of the incoming infrared light is detected thereby. Generally speaking, with this bolometer material, when the temperature coefficient of electric resistance (temperature coefficient of resistance is hereinafter referred to as “TCR”, and the following numerical value, small and large, and high and low of the TCR are all represented as absolute values excluding those in the drawings) increases, the noise equivalent temperature difference (NETD) of the infrared sensor decreases and the sensitivity will improve.
Today, a vanadium oxide thin film having a TCR of 2%/K is being used as a resistor film for a bolometer for use in an infrared sensor, but a material which indicates a higher TCR is being demanded. As such material that is indicative on a high TCR, a perovskite-type Mn oxide thin film is known. This thin film shows significant change in electric resistance, in the vicinity of 300K, associated with metal-to-insulator transition that is unique to this material, and a large TCR of roughly 10%/K can be obtained in the vicinity of 300K pursuant to the forgoing phase transition (Patent Document 1). Nevertheless, the TCR of 4%/K or higher is only indicated around ±5K in the vicinity of room temperature, and there is a problem in that the temperature range, for ordinary use as a bolometer thin film, is too narrow.
Meanwhile, various experiment and research have also been conducted with vanadium oxide films in order to achieve a higher TCR.
Non-Patent Document 1 describes that, when vanadium oxide is crystallized, a high TCR can be obtained pursuant to metal-to-insulator transition, and this results in hysteresis having a large resistance change in response to temperature variations. Nevertheless, if there is hysteresis having a large resistance change in response to temperature variations, it cannot be used as is as a bolometer thin film of an infrared sensor, and crystallized vanadium oxide that is indicative of a high TCR cannot ordinarily be used as a bolometer thin film. Thus, a film, in which an amorphous vanadium oxide film is produced through sputtering, and the electric resistance is controlled by partially reducing vanadium oxide thereafter, is used in an infrared sensor. The temperature variations of resistance in the foregoing case are semiconductive, and the TCR is low at roughly 2%/K.
Patent Document 3 describes a detector film for a bolometer having a crystallized vanadium oxide layer on an insulating substrate, wherein phase transition and hysteresis can be avoided by orienting the c axis of the vanadium oxide crystals in a direction perpendicular to the surface of the insulating substrate, and the foregoing crystalline orientation can be obtained stably by using a vanadium oxide layer containing nitrogen. Nevertheless, the TCR in the Examples is still a low value which is, at maximum, 2.79%/K. Accordingly, there is still no known vanadium oxide resistor film, which has a high TCR of 4%/K or higher in the vicinity of room temperature, and is free from large hysteresis in a resistance change in response to temperature variations.
With respect to a method of producing a vanadium oxide resistor film for use in a bolometer, Patent Document 1 describes that a solution of an organovanadium compound is applied on a support medium, laser light having a wavelength of 400 nm or less is irradiated onto this support medium after being dried, and the organovanadium compound is decomposed to obtain a crystallized vanadium oxide thin film.
Moreover, Patent Document 2 describes that the amorphous film is irradiated by ultraviolet laser in a reducing atmosphere, and a vanadium oxide resistor film, in which there is no hysteresis in temperature variations of resistance and the TCR is roughly 3%/K, can be thereby obtained so as to overcome the problem of the protracted treatment of reducing vanadium oxide, which is adopted for optimizing the electric resistance of the vanadium oxide resistor film.
Nevertheless, there is no conventional technology to produce, by irradiating laser light or ultraviolet light, a vanadium oxide resistor film, which has a high TCR of 4%/K or higher and is free from large hysteresis in a resistance change in response to temperature variations.